1. Field of Invention
This invention relates generally to toner compositions having improved properties that are provided by improved surface additives. More particularly, the present invention relates to toner and developer compositions where the toner particles have an external additive mixture of treated silica, treated titania, and calcium stearate. The invention also relates to developers containing such toners, processes for making such toners and developers, and methods for generating developed images with such toners and developers.
2. Description of Related Art
In electrophotography, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the photoconductor to a support, such as transparency or paper. This imaging process may be repeated many times.
Various toner compositions for such a printing system are well known in the art, and have been produced having a wide range of additives and constituent materials. Generally, however, the toner particles include a binding material such as a resin, a colorant such as a dye and/or a pigment, and any of various additives to provide particular properties to the toner particles.
U.S. Pat. No. 5,545,501 describes an electrostatographic developer composition comprising carrier particles and toner particles with a toner particle size distribution having a volume average particle size (T) such that 4 μm≦T≦12 μm, and an average charge (absolute value) pro diameter in femtocoulomb/10 μm (CT) after triboelectric contact with the carrier particles such that 1 fC/10 μm≦CT≦10 fC/10 μm, and wherein (i) the carrier particles have a saturation magnetization value, Msat, expressed in Tesla (T) such that Msat≧0.30 T; (ii) the carrier particles have a volume average particle size (Cavg) such that 30 μm≦Cavg≦60 μm; (iii) the volume based particle size distribution of the carrier particles has at least 90 percent of the particles having a particle diameter C such that 0.5 Cavg≦C≦2 Cavg; (iv) the volume based particles size distribution of the carrier particles comprises less than b percent particles smaller than 25 μm wherein b=0.35.times.(Msat)2×P with Msat: saturation magnetization value, Msat, expressed in T and P, the maximal field strength of the magnetic developing pole expressed in kA/m, and (v) the carrier particles comprise a core particle coated with a resin coating in an amount (RC) such that 0.2 percent w/w≦RC≦2 percent w/w, see the Abstract. This patent indicates that the developers thereof can achieve images when a latent image is developed with a fine hair magnetic brush, see for example, column 4, lines 7 to 17.
Nevertheless, there continues to be a need for a set of developers comprised of toners and carriers that possess a combination of properties such that when used to develop a latent image on the surface of a photoreceptor, preferably in an image-on-image device, and more specifically, in such a device also utilizing a hybrid scavengeless development system, the color image produced exhibits a quality analogous to that achieved in offset lithography. Further, there is a need for toners and developers wherein a toner additive does not substantially interact with fuser oils, fuser rolls, and the like to thereby, for example, increase the usable life, for example from about 200,000 prints to about 1,000,000 prints, of fuser devices, such as fuser rolls, and wherein the toners and developers thereof possess excellent triboelectrical, conductivity, and developability characteristics.
One approach for addressing these needs is shown in U.S. Patent Publication No. 20040063018, the entire disclosure of which is incorporated herein by reference. In the publication toner and developer compositions are disclosed that comprise at least one binder in an optional amount of from about 85 to about 99 percent by weight, at least one colorant in an optional amount of from about 0.5 to about 15 percent by weight, and calcium stearate in an optional amount of from about 0.05 to about 2 percent by weight.
Despite the various attempts to provide toner and developer compositions for providing high quality print results, problems still remain. For example, as the end-user demands for higher quality prints increases, and as the printing apparatuses are utilized in a wider variety of environments, increasing demands are being placed on the printing apparatuses and the toner and developer compositions. Thus, while particular printing apparatuses and toner and developer compositions are designed to provide adequate results over a wide range of customer job types and operating conditions, those parameters are being widened to increase the performance demands.
One such demand is the triboelectric charging values of the toner and developer compositions. The conventional range for developer triboelectric charging values is generally accepted to be from about 25 to about 50 μC/g. This range is limited on the lower end by macrouniformity (half-tone mottle), dirt, emissions, spittings, and gaps and traps defects; and is limited on the higher end by transfer image quality defects. While this range provides high quality print results for a large proportion of the end-users, there exists a “tail” of end-users at both ends, where operating conditions provide less than superior print results. These tail operation conditions are driven primarily by inherent variations in the machines and compositions as produced, as well as variation in the developer toner concentration, ambient temperature and relative humidity conditions, and age of the developer components. Relative humidity can be somewhat controlled, by using an environmental unit in the print cavity to dehumidify the print cavity and to control humidity on the high end, and a humidifier in the room housing the printing apparatus to control humidity on the low end. However, this requirement for the end-user to provide humidification control is difficult in some cases and thus is undesirable.